1. Field of the Invention
The present invention generally relates to time division multiplexed interferometric sensors. More specifically, the present invention relates to interrogating interferometric sensors in a manner that increases the allowable interrogation pulse duty-cycle and that improves signal-to-noise ratios.
2. Description of the Related Art
Time division multiplexing (TDM) of interferometric sensors is performed using pulsed light sources to produce pulse reflections from the sensors such that the reflections are distributed in time because of the different time delays associated with each sensor. The requirement that the reflections from the different sensors be separated in the time domain results in the need to use pulse duty cycles and pulse repetition rates that take into account the number of sensors being sensed and their separations.
An interferometric sensor may be described as being comprised of two paths from an interrogating transmitter unit to a receiver unit through an optical sensor network. The optical sensor network may comprise a number of paths, where certain pairs of paths form sensor interferometers. The different paths through the sensor network may typically be formed by optical waveguides and splitters like optical fibers, optical splitters, circulators, and other waveguide coupled components, or free space optical paths, mirrors, beam splitters and other bulk components. The time delay difference between the two paths of a sensor is called the imbalance of that sensor. The sensor imbalance can be made sensitive to some measurand that one wants to measure. Changes in the sensor imbalance are measured by extracting the phase of the interference between light components that has propagated the two paths as they are combined in the receiver. The extracted phase will thus provide information about the desired measurand. The portions of the sensor network that are common to both the sensor path and the reference path of a sensor may be called transport or lead paths. In a fiber optic sensor network the lead paths are called lead fibers.
Interferometric sensors can be multiplexed along the same fiber using time-division multiplexing (TDM). In TDM, the optical source outputs light with a periodic intensity pattern and with a repetition period T called the TDM repetition period. The duty-cycle of the source is defined as the fraction of time in which the source is turned on. The duty-cycle depends on the number of multiplexed sensors and the separation between the sensors. Each sensor directs a portion of the source light to the receiver. The sensors form different delays from the source to the detector, and signals directed from different sensors will therefore be separated in time at the detector.
A well-known time division multiplexed interrogation technique is the two pulse heterodyne sub-carrier generation technique as disclosed in J. P. Dakin, “An Optical Sensing System,” UK patent application number 2126820A (filed Jul. 17, 1982). The two pulse heterodyne technique repeatedly transmits two interrogation pulses that have pulse widths that are shorter than (or equal to) the sensor imbalance. The phase difference between the first and the second pulse of each pulse pair is linearly varied with time to produce a differential frequency shift between the sequences of first pulses and second pulses. In the two pulse heterodyne technique the second pulse that has propagated the shortest path of the interferometer and the first pulse that has propagated the longest path of the interferometer interfere, forming an interference pulse at the receiver which is detected and used for extraction of the sensor phase. The differential frequency shift between the first and second pulses of the pulse pairs produces a carrier frequency on the sequence of detected interference pulses. The phase of this carrier is extracted. This extracted carrier phase equals the sensor phase except for a constant phase term.
A well-known interrogation method for continuous wave (cw) interrogation of interferometric sensors is the phase generated carrier technique, disclosed in A. Dandrige et al., “Homodyne demodulation scheme for fiber optic sensors using phase generated carrier,” IEEE Journal of Quantum Electronics, 18(10):1647-1653, 1982. The phase generated carrier technique is based on a harmonic bias modulation of the sensor phase, for instance by modulation of the source phase, resulting in a detected interference signal that has signal components at harmonics of the source modulation frequency. The sensor phase (without the applied bias modulation) can be determined from a combination of the signal components of several harmonics of the source modulation frequency. This technique can also be used in combination with time-division multiplexing, see A. D. Kersey et al. “Time-division multiplexing of interferometric fiber sensor using passive phase-generated carrier interrogation,” Optics Letters, 12(10):775-777, 1987. The light source may then be pulsed in the same manner as for the two pulse heterodyne sub-carrier generation technique, while the source phase is modulated in the same manner as for the cw phase generated carrier technique. The detector is sampled at the arrival of the reflected pulses, and the sensor phase is calculated from the harmonics of the source modulation frequency.
One type of interferometric sensor is the inline Fabry-Perot sensor. When inline Fabry-Perot sensors are pulse interrogated, extra reflected pulses are received due to multiple reflections within the Fabry-Perot cavity. These pulses are called decay pulses. For Fabry-Perot interferometers, the number of decay pulses is in principle infinite. If for instance a decay pulse from sensor 1 arrives at the detector simultaneously with a detected interference pulse from sensor 2 that is used to calculate the sensor phase, the interference between the decay pulse and the detected interference pulse will introduce crosstalk from sensor 1 to sensor 2. Thus, the source duty-cycle and the delay separation between the sensors must be chosen so that the sequence of decay pulses has faded-out to a level that depends on the allowable crosstalk level. In the prior art, to suppress crosstalk between the inline Fabry-Perot sensors one set of decay pulses has to fade out before reflections from another pair of interrogation pulses can be received. Thus, most of the interrogation pulse duty-cycle is wasted by having to wait for the multiple reflections to fade. Other types of interferometric sensors have similar problems in that overlapping pulse reflections have to be prevented.
In view of the foregoing problems, an interferometric sensor interrogation method that increases the allowable interrogation pulse duty-cycle and that improves the signal-to-noise ratio would be beneficial.